DOS/4G and DOS/4GW FAQ:Mouse Callbacks

No. Although DOS/4G supports the Interrupt 31h interface for managing
real mode callbacks, you don't need to bother with them for their single
most important application: mouse callback functions. Just register your
protected mode mouse callback function as you would in real mode, by
issuing Interrupt 33h/0Ch with the event mask in CX and the function
address in ES:EDX, and you function will work as expected.

Because a mouse callback function is called asynchronously, the same
locking requirement exists for a mouse callback function as for a
hardware interrupt handler.